1. Field of the Invention
This invention relates generally to magnetic head servo control systems and, more particularly, to disk drive position control systems that determine the location of a head relative to disk tracks.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk system, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface in close proximity to the disk. That is, a transducer or read/write head is provided to produce and detect variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path over the centerline of the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data in the track. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo systems.
A servo control system generally maintains a read/write head in a position centered over a track by reading servo information recorded onto the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts formed as parallel radial stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The demodulated servo signal is referred to as a position error sensing (PES) signal.
In a sector servo method for providing servo track information to a disk servo control system, each disk surface of the disk drive includes servo track information and binary data recorded in concentric or spiral tracks. The tracks on a sector servo disk surface are divided into radial sectors having a short servo track information area followed by a customer data area. The servo track information area typically includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates to the data read/write head that servo information immediately follows in the track. The servo read head can be the same head used for reading data or can be a separate, dedicated servo head. The PES signal is used to generate a corrective input signal that is applied to the read/write head positioning servo.
FIG. 1 is a representation of a conventional quad-burst servo pattern in which magnetic transitions are recorded in bursts labeled as A, B, C, and D. The servo tracks are indicated by the track numbers along the left side of the drawing figure. The tracks extend across the page from left to right. The portion of the disk 22 shown in FIG. 1 extends approximately from servo track N-1.0 toward the inner diameter of the disk to servo track N+2.5 toward the outer diameter. Those skilled in the art will appreciate that position information is decoded by demodulating the signal generated by the head passing over the PES burst patterns to form a signal P based on: EQU P=A-B
and to form a quadrature signal Q based on: EQU Q=C-D
The signals P and Q are quadrature signals because they are cyclic and are out of phase by 90 degrees (one-quarter signal phase). The magnetic transitions that comprise the servo pattern are represented in FIG. 1 by vertical bars. The letter within each group of bars represents the servo burst recorded therein. One burst is distinguished from another by relative position in a track and relative position to the other bursts. Thus, for a servo head that can detect magnetic transitions from more than one track at a time, the signal P should be zero when the head is tracking exactly along the centerline of servo track N in FIG. 1, because when tracking along the centerline the head will receive equal amounts of magnetic field from the A and B servo pattern bursts. A similar situation exists for servo tracks N+1, N+2, and so forth. For servo track N+0.5, the signal Q should be zero when the head is tracking exactly along the N+0.5 track centerline because the head will receive equal amounts of field from the C and D servo pattern bursts. The signal Q should be zero also for servo tracks N+1.5, N+2.5, and so forth when tracking along their respective centerlines.
Conventionally, it is typical to use the same write element for recording the servo bursts and for recording customer data when the disk is thereafter used, and to use the same read element for reading servo bursts and for reading customer data recorded in the data tracks. The servo bursts of FIG. 1 are recorded in a trimming process, in which a portion of each servo burst, recorded so it extends into an adjacent track width on one servo writer step, is overwritten (erased) in the next servo writer step.
Assuming that "e" is an inaccuracy or error produced by each servo write step, and P and Q defined as above are produced by reading from adjacent servo bursts formed in different servo write steps with trimming, the P or Q inaccuracy can be approximated statistically as: EQU a=e*sqrt(2.0),
where e is the 3-sigma servo writer position error (tracking error) caused by one servo writer step.
FIG. 2 is an equivalent representation of the A, B, C, and D servo bursts of FIG. 1 arranged so that the servo tracks run vertically up and down the drawing sheet, the disk inside diameter (ID) is to the right side of the drawing and the disk outside diameter (OD) is to the left side of the drawing. Thus, the read/write head moves relative to the servo pattern from the top of the FIG. 2 illustration to the bottom of the FIG. 2 illustration. FIG. 3 is a graph of the P and Q values associated with the corresponding read/write head position relative to the FIG. 2 servo pattern. For example, if the read/write head is tracking exactly between the A, B bursts and through the mid-point of either the C or D bursts shown in FIG. 2, then the value of the P component of the PES signal should be zero because: EQU i P=(A-B),
and the value of the Q component of the PES signal should be at a maximum, because: EQU Q=(C-D).
This should be apparent from review of FIG. 2 and FIG. 3. Similarly, if the read/write head is tracking exactly between the C, D bursts and through the mid-point of either the A or B bursts, then the value of the Q component should be zero and the value of the P component should be at a maximum.
Thus, the P and Q signals are cyclic, having peaks and valleys, as the read/write head is moved laterally across the disk. This is how read/write head position can be determined from the PES signal. Unfortunately, due to readback head characteristics, the peaks and valleys of the corresponding PES signal will not necessarily provide a linear function with respect to the head position; rather, there are regions of nonlinear response. To ensure a more linear combined PES signal, it is typical to create a stitched PES signal by selecting between either the P signal or the Q signal, depending on which signal is in a linear operating range.
FIG. 4 shows the stitched PES signal formed by selecting between the P and Q values according to the following: EQU stitched PES=P*sgn(Q) if.vertline.Q.vertline.&gt;.vertline.P.vertline.; EQU else=Q*sgn(P),
where sgn(x) indicates the sign or polarity (positive or negative) of the value x. In this way, the switched PES signal comprises a signal formed by selecting the magnitude of either the P or the Q signal and then adjusting its polarity, depending on which is operating in its linear range. It should be apparent that the switched PES signal represents the linear portions of the respective P and Q signals shown in FIG. 3. The resulting stitched PES signal indicates the direction in which the read/write head should be moved to maintain the head centered between the corresponding track centers, either A and C or B and D.
There is a demand for ever-increasing amounts of storage capacity for customer data. One constraint on the amount of disk surface area for storing customer data is the amount of space required by the PES servo pattern itself, as shown in FIGS. 1 and 2. It should be appreciated that every bit of disk surface space freed from servo pattern usage can be shifted to customer data. It also should be appreciated that reducing the amount of error caused by the servo writer should permit high track density disk storage designs.
From the discussion above, it should be apparent that there is a need for a disk drive system with a servo pattern that increases the amount of disk surface area available for storage of customer data and reduces the time needed to produce the pattern. The present invention fulfills this need.